1. Field of the Invention
This invention relates to protective coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) formed of a zirconia-based ceramic material that exhibits improved strength and fracture toughness as a result of containing a dispersion of chromia particles or precipitates.
2. Description of the Related Art
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating ceramic topcoat, typically referred to as the TBC, typically bonded to the component with an environmentally-protective bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt). Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques.
TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation of the TBC. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.). In contrast, plasma spraying techniques such as air plasma spraying (APS) deposit TBC material in the form of molten xe2x80x9csplats,xe2x80x9d resulting in a TBC characterized by flat (noncolumnar) grains and a degree of inhomogeneity and porosity that reduces heat transfer through the TBC.
While YSZ TBC""s are widely employed in the art for their desirable thermal and adhesion characteristics, they are susceptible to mechanical damage within the hot gas path of a gas turbine engine. For example, YSZ coatings are known to be susceptible to thinning from impact and erosion damage by hard particles in the high velocity gas path. Impact damage and the resulting loss of TBC particularly occur along the leading edges of components such as turbine blades, while erosion is more prevalent on the concave and convex surfaces of the blades, depending on the particular blade design. Both forms of mechanical damage not only shorten component life, but also lead to reduced engine performance and fuel efficiency. In U.S. Pat. No. 5,683,825 to Bruce et al., an erosion-resistant TBC is disclosed in which alumina (Al2O3) or silicon carbide (SiC) is deposited as a protective coating on a TBC, or co-deposited with the TBC. Commonly-assigned U.S. patent application Ser. No. 09/710,682 to Rigney et al. discloses a TBC comprising alternating layers of YSZ and YSZ containing at least five volume percent of alumina and/or chromia (Cr2O3) particles to increase the impact and wear resistance of the TBC.
In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it must also maintain a low thermal conductivity throughout the life of the component. However, the thermal conductivity of YSZ has been observed to increase by 30% or more over time when subjected to the operating environment of a gas turbine engine. This increase has been associated with coarsening of the zirconia-based microstructure through grain and pore growth and grain boundary creep. As a solution, U.S. patent application Ser. No. 09/765,228 to Rigney et al. discloses a TBC formed to contain small amounts of alumina precipitates dispersed throughout the grain boundaries and pores of the TBC. These alumina precipitates getter oxide impurities that would otherwise allow or promote grain sintering and coarsening and pore coarsening, the consequence of which would be densification of the TBC and increased thermal conductivity. Another solution proposed in commonly-assigned U.S. patent application Ser. No. 09/833,446 to Rigney et al. is to modify a YSZ TBC to contain one or more additional metal oxides that increase crystallographic defects and lattice strains in the TBC grains and/or form precipitates of zirconia and/or compound(s) of zirconia and/or yttria and the additional metal oxide(s), all of which are disclosed as reducing the thermal conductivity of YSZ. Metal oxides disclosed as having this beneficial effect are the alkaline-earth metal oxides magnesia (MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO), the rare-earth metal oxides lanthana (La2O3), ceria (CeO2), neodymia (Nd2O3), gadolinium oxide (Gd2O3) and dysprosia (Dy2O3), as well as such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3).
In addition to compositional modifications such as those noted above, processing modifications are also being considered in order to reduce the thermal conductivity of YSZ. However, lower thermal conductivities correspond to a much higher thermal gradient through the thickness of a TBC, creating much higher stresses within the TBC due to the mismatch in coefficients of thermal expansion (CTE) between the ceramic TBC and the underlying metallic substrate and bond coat. It is possible that such stresses can exceed the fracture strength of a YSZ TBC. As such, TBC=s that exhibit improved strength and fracture toughness would be desirable for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) for a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The TBC of this invention exhibits improved strength and fracture toughness as a result of containing a dispersion of fine chromia particles or precipitates (hereinafter referred to simply as particles) within its porous microstructure. The TBC preferably consists essentially of yttria-stabilized zirconia and the chromia particles, which are dispersed throughout the microstructure of the TBC including the YSZ grains and grain boundaries. Importantly, the chromia particles are present in an amount sufficient to increase the strength and fracture toughness of the TBC, generally at least one volume percent up to about ten volume percent of the TBC. The resulting improved strength and fracture toughness of the TBC increases its ability to withstand high thermally-induced stresses that occur as a result of large thermal gradients through the TBC thickness.
In the form of discrete particles in the above-noted amounts, sufficient chromia is present to increase the strength and fracture toughness through dispersion hardening of the TBC material, while avoiding the presence of localized compositional gradients that would decrease the spallation resistance of the TBC. The dispersion of chromia particles serves to increase the strength and fracture toughness of YSZ, and therefore the entire TBC, more effectively than a discrete layer of chromia at the TBC surface. The presence of a uniform dispersion of chromia particles is also distinguishable from U.S. patent application Ser. No. 09/710,682 to Rigney et al., which discloses a TBC comprising alternating layers of YSZ and YSZ containing alumina and/or chromia particles. When present as a dispersion throughout the TBC (as opposed to layers), the chromia particles provide a uniform improvement in strength and fracture toughness throughout the life of the TBC.
Other objects and advantages of this invention will be better appreciated from the following detailed description.